I. Field of the Invention
This invention relates generally to papermaking, and includes an apparatus for deaerating an air-containing aqueous suspension of papermaking stock which is introduced into deaeration chambers or wings extending from a stock receiver. More specifically, the present invention is directed to a papermaking apparatus capable of providing a more constant hydrostatic head pressure to a papermaking headbox to improve the quality and consistency of the manufactured paper.
II. Background of the Invention
In the papermaking industry, an aqueous cellulosic paper stock is processed in preparation of ultimately being formed into paper by a device known as a headbox. The aqueous suspension of cellulosic papermaking stock is first cleansed of dirt and impurities, typically by hydrocyclones as a whirling annulus, and injected into elongated enclosures or wings. The hydrocyclones also facilitate deaeration by deatomizing the air molecules from the suspension by thrusting the suspension, which is formed into an annulus, against the inner wall of the deaeration wing chamber. Each of the deaeration wing chambers extend outwardly and upwardly from a large stock receiver. The chamber defined in each wing is in communication with the receiver interior such that the deaerated stock flows into the receiver to form a pool. Both the receiver and the wing chambers are maintained under a vacuum sufficient to effect deaeration of stock suspension sprayed into the wings, where the vacuum in the receiver chamber and wing chambers is substantially equal.
The pool of deaerated stock in the receiver creates a hydrostatic head. The deaerated paper stock is withdrawn from the pool through a lower port and is communicated to the papermaking headbox. The headbox subsequently manufactures paper from the deaerated and cleansed paper stock. The quality and caliper of the manufactured paper is dependent upon many factors including the quality of the papermaking stock, as well as how constant the hydrostatic head created by the pool of paper stock suspension is maintained. It is crucial that the hydrostatic head be maintained as constant as possible to reduce headbox vibration, and pressure fluctuations, both of which can degrade the quality of paper manufactured by the headbox. The more constant the pool level of deaerated stock is maintained in the receiver chamber, the more constant the resulting hydrostatic head, and hence, the higher the quality and consistency the resulting paper product.
U.S. Pat. No. 3,206,917 to Kaiser which is assigned to Clark & Vicario Corporation addresses the problem of maintaining a substantially uniform hydrostatic head of deaerated paper stock. This invention teaches a weir which is positioned in the receiver chamber to define both a pool of deaerated paper stock, and an overflow chamber. Deaerated paper stock in excess of the amount needed to provide a predetermined hydrostatic head overflows from the pool into the overflow chamber, and is recycled and further cleansed of dirt and other impurities. A generally uniform hydrostatic head results from the pool in the receiver when the level of deaerated stock matches the height of the weir. However, the pool level in the receiver is still subject to variations due to both a standing wave and splashing generated by the deaerated stock rushing from each of the inclined wings into the receiver. Deaerated stock comes rushing down each of the wings into the receiver agitates the surface of the pool such that resulting fluctuations in the hydrostatic head are generated. These variations in the hydrostatic head are sensed by the papermaking headbox. Hence, even when employing a weir, the consistency and quality of the paper manufactured by the headbox is subject to degradation.
A papermaking apparatus with reduced fluctuations of the hydrostatic head created by the deaerated paper stock, including apparatuses which have weirs, is desirable to improve the quality and consistency of the manufactured paper. Further, reducing the variations of the pool level, including standing waves and splashing, will reduce the amount of deaerated paper stock which overflows into the overflow chamber, thus reducing the amount of paper stock which needs to be subsequently recleansed and further deaerated. The reduction of further processing the overflow paper stock reduces the amount of expensive deaerating chemicals required as well.